1. Field of the Invention
The present invention relates to a palladium plating solution and, more particularly, to a palladium plating film which provides a junction with a junction material, such as a solder.
2. Description of the Related Art
In recent years, as electronic parts and semiconductor parts there have been various things such as a printed circuit board and a package. Examples of what are called packages include a lead frame, a BGA (ball grid array), an LGA (land grid array package), a QFP (quad flat package), a minimold package and the like. To meet requirements for high-density packaging, these packages have been improved day by day to adapt to miniaturization and multi-pin designs and characteristics required of them tend to be increasingly severe.
In such electronic parts and semiconductor parts, solders and bonding wires have hitherto been used as their junction materials, and soldering and wire bonding have been established as junction techniques indispensable for mounting packages on printed circuit boards, such as printed wiring boards.
With respect to the packaging techniques of such electronic parts and the like, in the case of a lead frame, in order to improve the junction characteristics of a wire bonding terminal and a solder bonding terminal, there has been known a technique which involves forming a junction composed of a nickel plating film, a palladium plating film and a gold plating film on a copper surface constituting these terminals (refer to Japanese Patent Application Laid-Open No. 9-8438).
The reason why a palladium plating film is adopted in such junctions is that this palladium plating film can prevent the diffusion of the copper of a base sheet into nickel, thereby ensuring the junction by soldering and wire bonding. However, as a result of the recent progress in miniaturization and high-density packaging techniques for electronic parts and semiconductor parts, characteristics required of this junction have become more and more severe than ever.
For this reason, there has been proposed a technique which involves forming a junction in the outer lead of a lead frame by causing elements such as tellurium and antimony to be contained in this palladium plating film (refer to Japanese Patent Application Laid-Open No. 6-232311). Such a palladium plating film can maintain good solderability even when a junction is subjected to a thermal history to a certain degree.